Tocopherols are naturally occurring phenolic compounds and are the major forms of vitamin E in our body. Tocopherols exist in four forms, designated as alpha (?), beta (), gamma (?), and delta (?). Recent clinical trials with ?-tocopherol have provided disappointing results for cancer prevention. We believe hese studies were designed based on insufficient knowledge of the cancer preventive activities of the different forms of the tocopherols. There is an urgent need for investigating the activities of the different forms, such as ?-tocopherol, the major form of vitamin E in the diet in the U.S, and for identifying molecular mechanisms and targets of tocopherols. Our recent studies demonstrate that tocopherol mixtures that are rich in ?-tocopherol (?-TmT) inhibit progression of mammary hyperplasia and induce apoptosis in vivo and in cell lines, and prevent mammary tumorigenesis in animal models of breast cancer. We hypothesize that ?- and ?-tocopherols suppress oxidative/nitrosative stress, modulate estrogen receptor (ER) and peroxisome proliferator-activated receptor-? (PPAR?) signaling, inhibit cell proliferation and induce apoptosis, resulting in the inhibition of breast carcinogenesis. We propose to test our hypotheses and achieve our objectives by addressing four Specific Aims: 1. Investigate the dose-dependent inhibitory effects of a naturally occurring tocopherol mixture ?-TmT on estrogen-induced hyperplasia and tumorigenesis in the ACI rat model of mammary cancer. 2. Purify individual tocopherols (T), ?-T, ?-T, and ?-T, and determine their cancer preventive activities in estrogen-mediated mammary tumorigenesis in ACI rats. 3. Elucidate the molecular mechanisms of action of tocopherols in modulating estrogen receptors (ERs) and PPAR?, resulting in suppressing cell proliferation and cell survival in cell lines. 4. Investigate the molecular mechanisms of action of tocopherols in suppressing oxidative/nitrosative stress, and the possible involvement of NF-E2 related factor-2 (Nrf2). Tocopherols are commonly occurring in vegetable oils and readily available as dietary supplements. Understanding the effects of naturally occurring tocopherol mixtures and individual tocopherols and identifying the key mechanisms of action is critical for their use in prevention of cancer. Our data will be valuable for future studies in selecting the optimal form or mixtures of tocopherols and in designing better protocols for human breast cancer prevention trials. This study will provide the first comprehensive evaluation of tocopherol mixtures and individual tocopherols for the prevention of breast cancer.